Objective: Therapeutic alliance has been associated with better treatment engagement, better adherence, and less dropout across various treatments and disorders. In treatment of posttraumatic stress disorder (PTSD), it may be particularly important to establish a strong early alliance to facilitate treatment adherence. However, factors such as childhood sexual abuse (CSA) history and poor social support may impede the development of early alliance in those receiving PTSD treatment. We sought to examine treatment adherence, CSA history, and social support as factors associated with early alliance in individuals with chronic PTSD who were receiving either prolonged exposure therapy (PE) or sertraline. Method: At pretreatment, participants (76.6% female; 64.9% Caucasian; mean age = 37.1 years, SD = 11.3) completed measures of trauma history, general support (Inventory of Socially Supportive Behaviors), and trauma-related social support (Social Reactions Questionnaire). Over the course of 10 weeks of PE or sertraline, they completed early therapeutic alliance (Working Alliance Inventory) and treatment adherence measures. Results: Early alliance was associated with PE adherence (r = 0.32, p less than 0.05) and overall treatment completion (r = 0.19, p less than 0.05). Only trauma-related social support predicted the strength of early alliance beyond the effects of treatment condition ([beta] = 0.23, p less than 0.05); CSA history was not predictive of a lower early alliance. Conclusions: Given the associations with adherence, clinicians may find it useful to routinely assess alliance early in treatment. Positive trauma support, not CSA history, may be particularly important in the development of a strong early therapeutic alliance. (Contains 4 tables and 3 footnotes.)